The present invention relates to a stencil printer and more particularly to a press drum included in a stencil printer.
A stencil printer including a rotatable ink drum and a rotatable hollow cylindrical press drum is conventional. While the ink drum is rotated with a master wrapped therearound, the press drum is rotated while pressing consecutive papers against the ink drum one by one. As a result, ink fed from ink feeding means is transferred to each paper in order to print an image thereon.
Usually, the press drum has substantially the same diameter as the ink drum and is driven to rotate at the same speed as the ink drum. A recess is formed in a part of the outer periphery of the press drum in the axial direction of the press drum. Clamping means for clamping the leading edge of a paper or similar recording medium is positioned in the recess. A master clamper for clamping a master is mounted on the ink drum. The recess is positioned in relation to the master clamper so as to prevent the press drum from interfering with the master clamper and to reduce the displacement of the press drum and therefore noise when the press drum is pressed against the ink drum. The press drum conveys a paper with the clamping means clamping the leading edge of the paper. This prevents the paper being discharged from curling and enhances accurate registration.
One of conventional press drums includes an aluminum molding formed by extrusion molding and having a partly removed circular section. A rubber layer or elastic layer is formed on the outer periphery of the molding and has its periphery ground. The rubber layer is made as thin as, e.g., 3 mm to 5 mm in order to reduce the weight of the press drum. Therefore, even if rubber constituting the rubber layer has a rubber hardness of HS20 as prescribed by JIS-A, the layer deforms to a degree corresponding to a rubber hardness of HS40 as also prescribed by JIS-A due to its thinness. This is because the rubber layer is too thin to exhibit its elasticity corresponding to the hardness selected. For example, assume that images are printed on envelopes. Then, when images are continuously printed on several hundred envelopes, a master tears in its portions corresponding to the comparatively thick portions of the envelopes. As a result, ink deposits on and smears the successive envelopes via the torn portions of the master.
In light of the above, the hardness of the rubber layer may be reduced in order to reduce the concentration of a pressing force on the comparatively thick portions of a paper. This kind of scheme, however, causes the rubber layer to perform bulk movement to both sides of a paper. As a result, the distance between the center of the press drum and a pressing position becomes equal to the sum of the radius of the press drum and the thickness of the paper and therefore greater than the distance between the center of the ink drum and the pressing portion, as will be described specifically later. Consequently, the paper moves at a higher linear velocity than the periphery of the ink drum, as seen at the printing position, causing the master to crease.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 5-330225, 8-332769, 9-1914, 9-216448 and 8-58216 as well as in U.S. Pat. No. 4,911,069.